Plasticisers are incorporated into a resin (usually a plastic or elastomer) to increase the flexibility, workability, or distensibility of the resin. The largest use of plasticisers is in the production of “plasticised” or flexible polyvinyl chloride (PVC) products. Typical uses of plasticised PVC include films, sheets, tubing, coated fabrics, wire and cable insulation and jacketing, flooring materials such as vinyl sheet flooring or vinyl floor tiles, adhesives, sealants, inks, and medical products such as blood bags and tubing, and the like.
Other polymer systems that use small amounts of plasticisers include polyvinyl butyral, acrylic polymers, poly (vinyldiene chloride), nylon, polyolefins, and certain fluoroplastics. Plasticisers can also be used with rubber (although often these materials fall under the definition of extenders for rubber rather than plasticisers). A listing of the major plasticisers and their compatibilities with different polymer systems is provided in “Plasticisers”, A. D. Godwin, in Applied Polymer Science 21st Century, edited by C. D. Craver and C. E. Carraher, Elsevier (2000); pp 157-175.
Plasticisers can be characterised on the basis of their chemical structure. The most important chemical class of plasticisers is phthalic acid esters. Two other important chemical classes are adipic acid esters and trimellitic acid esters. Di- and tri-esters of these aforementioned acids, having a molecular weight range from about 300 to 600, typically offer a balance of solvency and compatibility with the resin, yielding plasticised materials with useful properties and good aging abilities.
The performance specification for PVC formulations employed for wire and cable coating or jacketing depends upon the nature of the wire and cable and the use to which it is to be put. For example, European domestic wiring is subject to less rigorous conditions than wire and cable in automobiles which may be subject to a wide range of temperatures. Typically, wire and cables in automobiles are required to withstand temperatures as high as 150° C. due to the heat generated by the passage of electricity through the cable and as low as −40° C. to ensure the insulation is preserved if the automobile is subject to extreme weather conditions. Low temperature performance is also required for cables such as outdoor extension cables that are used in cold climates.
Important properties of a plasticiser for electrical insulation products include without limitation high plasticising efficiency, excellent compatibility with the resin, excellent processability, excellent oxidative stability, very low conductivity, and low volatility. Usually, when changes are made to improve one of these properties, some other important property is adversely affected. For example, an increase in the molecular weight of the alcohol used to produce the ester tends to reduce volatility at the expense of plasticising efficiency; the use of lower molecular weight alcohols will improve plasticising efficiency, but it can make the plasticizer too volatile for some applications. In addition, as the molecular weight of the ester plasticiser increases, its compatibility with PVC decreases, eventually resulting in a less desirable flexible PVC product with limited potential.
It is known to use di-tridecyl phthalates such as Jayflex™ DTDP for PVC wire and cable coating formulations to obtain the combination of low volatility and improved high temperature performance.
An important factor in the selection of the insulating material for wire and cable coating, and, in particular, automotive wire and cable coating, is the temperature to which the wire and cable is to be subjected (sometimes known as the service temperature). Cables are currently classified from T1 to T6 or Class A to Class D with service temperatures between −40° C. and 150° C. T1 or Class A being the mildest classification and T6 or Class D being amongst the most stringent. The low temperature performance is determined by cold strength tests, dynamic lending strength tests, or by wrapping (mandrel) tests according to ISO 6722. Linear phthalates such as di-linear-undecyl phthalate (Jayflex™ L11P or Palatinol® DUP) are known to improve low temperature properties compared with branched phthalates such as di-tridecyl phthalate (Jayflex™ DTDP). However, linear phthalates require linear alcohols which are scarce and expensive and accordingly the less expensive di-tridiceyl phthalates have been preferred. However, DTDP does not satisfy the more extreme low temperature requirements, particularly for cables of large cross sectional area. PVC wire and cable coating formulations are required to have good abrasion resistance to resist removal of the insulation. Abrasion resistance improves as the amount of plasticiser employed reduces; however, employing lower levels of plasticizers results in poorer low temperature flexibility.
The range of alcohols that are commercially useful in esterification for phthalate ester plasticiser manufacture is generally limited from about C4 to about C14 monohydric alcohols. It is known that the specific alcohols from which the esters are made influence the performance properties, e.g., the size and structure of the alkyl group helps determine the volatility and gellation temperature of the plasticisers. The type of alcohol used for esterification is therefore chosen according to the application in which the plasticised polyvinyl chloride is to be used. The alcohols from which the plasticiser esters are made are generally obtained by either olefin oligomerisation followed by hydroformylation or by hydroformylation of olefins to form aldehydes, followed by aldehyde dimerisation, generally through an aldol reaction. The alkyl groups of the esters therefore vary in size and structure according to the process used to produce the alcohols.
It is known to blend plasticisers to obtain a desired combination of properties. U.S. Pat. No. 6,969,735 relates to the use of a mixture of phthalates and trimellitates to plasticise PVC, to produce a composition useful for wire and cable coating and jacketing. Specifically, a plasticiser blend comprising 43 parts of di-tridecyl phthalate and 7 parts of tri-isononyl trimellitate is used per 100 parts of PVC. These blends are said to have improved high temperature performance. However, trimellitates are known to exhibit poorer cold flexibility properties than phthalates. Therefore, a blend with phthalates would fail to meet the cold strength test.
As the linearity of the alcohol used to make the phthalate ester increases, certain predictable events occur. One may expect reduced plasticiser volatility, improved plasticiser efficiency towards making PVC flexible, improved low temperature flexibility, and sometimes improved processability, the latter characteristic being often a combination of plasticiser solvency and plasticiser neat viscosity. Linearity is usually defined by either a branching index or by the number of branches per alkyl side chain as determined by proton NMR analyses. As the linearity of a plasticiser increases, its compatibility with PVC can decrease, where “compatibility” is used to reference a usable product with no or slight plasticiser exudation under stress. As linearity of a plasticizer increases its diffusion within the PVC is higher and its migration from PVC to another material in contact with PVC increases. For trimellitate esters, the most common alcohols used to prepare the ester are C8 through C9 primary alcohols. However, the use of more linear alcohols increases the cost of the plasticiser.